Just click here or on the image below to open the printable map—then bring outside!

This month’s highlight: The Farthest Thing You’ve Ever Seen

A common astronomy question is “what’s the farthest thing I can see using just my eyes?” Today we know the answer, but less than 100 years ago, no one had any idea. It wasn’t until 1925 that Edwin Hubble (yes, THAT Hubble) and his colleagues figured it out, changing our place on the cosmos (and the science of astronomy) forever.

Determining the true scale of the universe is a problem that confounded many generations of astronomers and physicists. The solution came in a series of painstaking steps playing out over thousands of years, each new milestone building on those which came before. The ancient Greeks made the first breakthroughs by determining the diameter of the Earth and eventually the approximate distance to the Moon.

Two millennia later, in the 17th and 18th centuries, the next great leap occurred. Careful observations of the planets Mars and Venus allowed astronomers to calculate the distance to the Sun with reasonable accuracy. Knowing that, the distances to the planets could be determined, and the size of our solar system came into focus.

The next challenge was the stars. How far away were those tiny points of light? Improvements in instrumentation allowed 19th century astronomers to measure the distances of a few dozen stars. Then it was discovered that a certain type of variable star pulsates (changes its brightness) on a schedule that depends on its brightness. By carefully measuring the pulsations of these special stars, their distances could be known.

Which brings us to the early 20th century. Astronomers now knew the distances to many stars, and knew that billions of stars were collected into our galaxy, the Milky Way. It seemed we might at last have a handle on the size of the universe. But the universe had other ideas. The problem was the nebulae.

Scattered among the stars in their telescopes, astronomers spotted hazy patches of light that became known as nebulae—the Latin word for “mists.” The nature of the nebulae was unknown. They looked like misty clouds of dust or gas, perhaps illuminated by nearby stars. As ever larger telescopes were constructed, researchers discovered that many nebulae were comprised of exquisitely faint stars, but most astronomers still assumed the nebulae were within our own Milky Way.

In 1925, Edwin Hubble turned the largest telescope of the day on the so-called Andromeda Nebula. He was able to measure some of the special pulsating stars in the nebula. When he performed the distance calculations, the results were astounding. The Andromeda Nebula was not in the Milky Way after all. It was a separate galaxy, the Andromeda Galaxy, and it was far beyond the farthest extremes of the Milky Way. In one fell swoop, the true size of the universe was revealed, and it was incomparably larger than previously imagined.

Which brings us to this month’s Sky Map. Find a dark location far from city lights. Spend 30 minutes enjoying the various sights on the map while your eyes become fully dark adapted. Now you’re ready. Between the Great Square of Pegasus and the Big “W” of Cassiopeia, look for a small hazy cloud. That’s the Andromeda Galaxy, the “nebula” that allowed Edwin Hubble to determine the scale of the cosmos. And it’s the farthest thing you’ll ever see with your eyes alone.